I'm not too sure. I think its for a few reasons. They don't want people who have a small plane approach ingrained in them, if they have accumulated 1500+ hours then why has no one recruited them etc.
I know that when I researched this area, I spoke with loads of pilots in charge of recruiting and they nearly all came back with the same point. Ideally no more than about 1000hours instructing.
But I think this demonstrates my point, that all airlines are different and the trick is to highlight the strengths that are particular to that specific airline.
Good luck